Hi,
Doesn't matter for now, the idea is to shunt the cathode resistor to ground and by the same token to improve the noise rejection.
Just give it a try ( if you don't mind) to see if you like what you hear.
Ciao, 😉
Doesn't matter for now, the idea is to shunt the cathode resistor to ground and by the same token to improve the noise rejection.
Just give it a try ( if you don't mind) to see if you like what you hear.
Ciao, 😉
I have some 47uF Elna Cerafines i was thinking bypassing with 10uF ERO MKTs. If i like it, i will probably get some MKP's.
When i'm thinking about it, same in my Yaqin mc100 i think, but there is 470uF parallell with 1,2K for an 12AX7. And i have a Cerafine there actually too.
When i'm thinking about it, same in my Yaqin mc100 i think, but there is 470uF parallell with 1,2K for an 12AX7. And i have a Cerafine there actually too.
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I have also made this preamplifier for a friend. I have AC-filaments with center tapped transformer. Should I use voltage divider to elevate filaments? I don't have any hum. Also I have no grid stopper resistor. Should I have? I haven't noticed any HF oscillation.
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It is good practice to elevate your heaters slightly above the cathode potential - generally 20V DC. The heaters should at least be grounded. if you haven't got hum then its not an issue.
Grid stoppers are a safety net for if the preamp gets operated in a Radio Frequency noisy environment, or for certain valves it prevents the preamp becoming a radio transmitter. Radio interference can be present and go undetected but will impart a hard edge to the sound. Its just good practice to include grid stoppers.
Thats the same signal cable I used on my phono preamp. excellent quality - but rather significant amounts of capacitance.
Shoog
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With SRPP topology, top triode cathode is at about +B/2, down triode cathode is at near 0 V, so to protect the valve to not exceed max Vkf, you must elevate heaters at about +B/4
Usually, SRPP topology don't need grid stoppers, especially in the top triode, with high transconductance valves, you can put a grid stopper in the down triode, try values as small as possible because resistor thermal noise is injected directly in the valve.
If you haven't HF oscillations, then you don't need grid stoppers.
Thank you for answers, Shoog and popilin, I will elevate the heaters. I haven't noticed any oscillations on my 20MHz scope but just to be on the safe side, I will try some 100 to 330R grid stoppers on the lower triode.
In my preamp i hear my cellphone clearly when it's searching for signal (where i live i have only 50% signalstrenght)
Maybe i will try 100R instead for gridstopper.
To my finnish friend; where did you get that transformer?
Maybe i will try 100R instead for gridstopper.
To my finnish friend; where did you get that transformer?
Max, I purchased PT from Uraltone, Helsinki. It's Hammond, model is 370BX if I recall correctly: Hammond 370BX verkkomuuntaja - Hammond 300-sarja - Verkkomuuntajat - Muuntajat ja kuristimet - Elektroniikan komponentit
My friend thinks that it is lot better than his solid state pre, but not as good as Conrad/Johnson he had in in loan.
My friend thinks that it is lot better than his solid state pre, but not as good as Conrad/Johnson he had in in loan.
Good, but in the bottom triode only, top triode has already a "natural" grid stopper, for unbypassed SRPP version
Rgg2.u= Ra1+(1+µ1)Rk1
For bypassed SRPP version
Rgg2.b=Ra1
Maybe your preamp is picking RF from another way as well as hum.
Is the RF even with the case closed?
Have not tested with lid on. But probably be better with.
What would the difference be between a 100 or 330ohm gridstopper? I don't have any 100ohm now i think, only 150ohm. There is a gridstopper already Kiwame 330ohm
Doing some simulation in Duncan PSU now. Uploading in a minute...
After reading little John Broskie and thinking about what i've hopefully learned here i came up with this after many simulations.
The first 100ohm is to make a calm life for rectifier, the others are part of a cascade filter. In sim i bundled 2x10uF MKP together thus 20uF.
What do you all think?
Also i measured my trafo and choke for exact values.
Thanks to Carl for simhelp.
The first 100ohm is to make a calm life for rectifier, the others are part of a cascade filter. In sim i bundled 2x10uF MKP together thus 20uF.
What do you all think?
Also i measured my trafo and choke for exact values.
Thanks to Carl for simhelp.
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Not much difference, but 330 ohm could be as much as equivalent noise resistance of triode, then you will double the triode noise.
Your preamp already have a first order low pass filter made by the volume pot and valve capacitance.
http://frank.pocnet.net/sheets/113/6/6N3P.pdf
Is it possible to see in datasheet? You think it would be better to go under 330R to avoid that problem?
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